1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine.
2. Description of Related Art
A previously proposed technique will be described with reference to FIG. 8 (some reference numerals used in FIG. 8 are common to those described in the following embodiments).
Hereinafter, a valve timing control system, which changes opening and closing timing of at least one of an intake valve(s) and an exhaust valve(s) of an internal combustion engine, will be also referred to as a variable valve timing control system and will be denoted as a VVT system. A previously proposed VVT system shown in FIG. 8 includes a variable valve timing mechanism 2, a hydraulic control system and an electronic control unit (ECU) 3. The variable valve timing mechanism 2 is also referred to as a variable camshaft timing mechanism 2 and will be denoted as a VCT mechanism 2. The VCT mechanism 2 can linearly change the opening and closing timing of the valve. The hydraulic control system hydraulically controls the operation of the VCT mechanism 2. The ECU 3 electrically controls a phase control valve 22, which is provided in the hydraulic control system. The phase control valve 22 will be also referred to as an oil control valve 22 and will be denoted as an OCV 22.
The VCT mechanism 2 includes a housing rotor 4 and a vane rotor 5. The housing rotor 4 is driven to rotate by the crankshaft of the engine. The vane rotor 5 drives a camshaft of the engine. The vane rotor 5 is rotated relative to the housing rotor 4 by a hydraulic pressure difference between a hydraulic pressure of advance chambers A and a hydraulic pressure of retard chambers B to adjust an amount of advance of the camshaft relative to the crankshaft.
Here, the camshaft is used to drive the intake valve(s) or the exhaust valve(s) to open and close the same, so that the torque fluctuation is generated in the camshaft at the time of opening and closing the valve(s).
The torque fluctuation of the camshaft is transmitted to the vane rotor 5, so that the vane rotor 5 shows the torque fluctuation toward the retard side and the advance side relative to the housing rotor 4.
When the torque fluctuation applied to the vane rotor 5 is increased toward the retard side, a force acts on the hydraulic pressure of the advance chambers A to discharge the hydraulic pressure from the advance chambers A. In contrast, when the torque fluctuation applied to the vane rotor 5 is increased toward the advance side, a force acts on the hydraulic pressure of the retard chambers B to discharge the hydraulic pressure from the retard chambers B. The torque fluctuation toward the retard side is larger than the torque fluctuation toward the advance side.
Thus, when the hydraulic pressure supplied to the advance chambers A is increased from a low hydraulic pressure state of the advance chambers A (a retarded state) to change the phase of the camshaft from the retard side to a target phase on the advance side, the vane rotor 5 is pushed backward toward the retard side due to the torque fluctuation, so that the response time, which is required to reach the target phase, is disadvantageously lengthened, as shown by a dotted line in FIG. 9.
In order to address the above disadvantage, it has been proposed to provide an advance check valve 23 in an advance fluid passage 31, which conducts the hydraulic pressure from the OCV 22 to the corresponding advance chamber A, to permit the hydraulic fluid to flow from the OCV 22 to the advance chambers A while limiting the hydraulic fluid to flow from this advance chamber A to the OCV 22 (see, for example, Japanese Unexamined Patent Publication No. 2006-46315 that corresponds to U.S. Pat. No. 7,182,052).
When the advance check valve 23 is provided, the vane rotor 5 is not pushed backward toward the retard side by the torque fluctuation at the time of changing the phase of the camshaft from the retard side to the target phase on the advance side, as indicated by a solid line in FIG. 9 to improve the response in the advance operation.
In contrast, when the phase of the camshaft is changed from the advance side to the tart phase on the retard side, the hydraulic pressure of the advance chambers A needs to be drained while bypassing the advance check valve 23. In view of this, in Japanese Unexamined Patent Publication No. 2006-46315, an advance drain control valve 25, which opens and blocks an advance check valve bypass passage 24, is provided.
The advance drain control valve 25 of Japanese Unexamined Patent Publication No. 2006-46315 is an opening/closing valve, which uses the hydraulic pressure supplied from the OCV 22 to the advance chamber A as a pilot hydraulic press. When the hydraulic pressure, which is supplied from the OCV 22 to the advance chamber A, is increased, the advance drain control valve 25 blocks the advance check valve bypass passage 24. In contrast, when the hydraulic pressure, which is supplied from the OCV 22 to the advance chamber A, is decreased, the advance drain control valve 25 opens the advance check valve bypass passage 24 due to action of a spring to drain the hydraulic pressure from the advance chamber A.
As discussed above, in the above technique, the hydraulic pressure, which is supplied from the OCV 22 to the advance chamber A, is used as the pilot hydraulic pressure of the advance drain control valve 25. Thus, in the case where the phase of the camshaft is changed from the retard side to the target phase on the advance side, when the hydraulic pressure of the advance chambers A is fluctuated (pulsed) by the torque fluctuation applied from the camshaft to the vane rotor 5, a valve element of the advance drain control valve 25 is fluctuated by the pressure pulsation. Therefore, the advance check valve bypass passage 24, which needs to be blocked, is repeatedly opened and closed. This may possibly deteriorate the response in the advance operation.
In order to address the above disadvantage, it is conceivable to provide a drain switch valve 29, which controls the pilot hydraulic pressure of the advance drain control valve 25, as shown in FIG. 8. The drain switch valve 29 is also referred to as an oil switching valve 29 and will be denoted as an OSV 29. Here, it should be noted that the provision of the OSV 29 in the manner shown in FIG. 8 should not be considered as a prior art.
The OCV 22 and the OSV 29 need to be operated synchronously.
However, when the OSV 29 is provided separately from the OCV 22, a performance of an electric actuator (e.g., a solenoid actuator) of the OCV 22 and a performance of an electric actuator (e.g., a solenoid actuator) of the OSV 29 may differ from one another, or a variation may occur in applied electric current, so that the OCV 22 and the OSV 29 may not precisely synchronized in some cases.
Furthermore, when the OSV 29 is installed separately from the OCV 22, the mounting flexibility may be deteriorated.
Also, when the OSV 29 is installed separately from the OCV 22, the number of components is increased to cause an increase in the cost.